His cum

"Well..." Jake said with a tilting his head towards Seth's rebellious crotch. Seth looked down, saw his peeping peepee and with with wide eyes shouted, "MY DICK?! WHAT THE FUCK?!? YOU WANNA CUT IT OFF LIKE THAT EPISODE OF RICK AND MORTY?!?!?! WHAT'RE YOU GONNA DO WITH IT? TURN IT INTO AN ALIEN KIDNEY OR SOMETHING?!??!?!?!?!?!?!?"

"No... No, Seth, calm down." the sexy clone said in a frighteningly perfect imitation of the irritation and exasperation of the 'original' Jake when he used to try to explain why Seth's assignment was dog shit. "They don't wanna cut anything off or do anything nasty to you. See, all they want is your semen."

"My... what?! Why do they want that, are they gonna make alien babies or something?!?!" he replied with hardly any reduction in shock.

"Hehe... no" came the laughing response, "They don't wanna make any 'alien babies', that would just be weird."

"Then what do they want it for?"

"Well..."

After a few moments of hesitation and some scared, semi-aroused gesturing from Seth, Jake continued,

"Well... you see... these aliens are kinda... really... curious."

"Huh?"

"See, humans are a bit of an enigma to them. They evolved to move passed the need for reproduction of any kind, so to them, sex is really just a nasty, primitive reproductive ritual. But you guys... humans I mean... have turned it into the weirdest form of art in the universe. Sure, other species write poetry and make music and paint sculptures, and go to war over what it means, but you guys are the only ones who do all that over mating! Like you guys have petabytes of porn available to you, but you also think you shouldn't watch it, it's crazy!"

"So they want my cum becaaauussse..."

"They wanna know what effect sex has on you - an objectively cute, but under-fucked, gay twink. On which point..." Jake stood up, smoothed out his clothes and started walking towards Seth, "There is one thing that you have to do before you get to decide whether you wanna stay or go..."